Alternating current (AC) signals such as those from inductive or capacitive sensors benefit greatly from the use of phase sensitive synchronous demodulation to obtain amplitude information. This method provides a high degree of filtering of extraneous noise outside the frequency and phase of the required signal. A typical input signal chain is compromised of three main elements: synchronous demodulator, filter and analog to digital converter. Although there are several variations, the classic method of implementing a demodulator is to use a switch controlled by the reference oscillator. This switch switches alternately between the input signal and its inverse to produce a synchronously rectified result. The phase timing of this switch is set to produce a full or half wave rectified version of the AC signal. Phase adjustment maybe included to enhance signal fidelity such as removing unwanted phase shifts in the signal path. Switching must be fast, exact and the effects of switch resistance or amplitude imperfections may degrade overall system performance.
Filtering of the rectified signal has to maintain fidelity and bandwidth for the signal of interest, while ripple must be minimized, consequently the analog filter circuit is often complex and limits the overall performance of the measurement system.
Alternate methods have been used where a digital signal processor is used to digitize the AC signals and then perform demodulation and filtering in software. This overcomes some of the filtering limitations but in doing so requires the use of more complex and costly high speed digital to analog converters and digital signal processor (DSP) hardware.